For example, a thermal flow meter of a constant temperature driving type is configured to control voltages that are to be applied so as to make the temperatures of electrical resistive elements respectively provided on the upstream side and downstream side of a flow path constant, and on the basis of the upstream side voltage and the downstream side voltage at the time, calculate a flow rate of fluid flowing through the flow path. More specifically, as given in Expression 1, the flow rate is obtained from a sensor output that is obtained by dividing a voltage difference, which is the difference between the upstream side voltage and the downstream side voltage, by the sum of the upstream side voltage and the downstream side voltage.
[Expression 1]Q=Sens((Vu−Vd)/(Vu+Vd))  (1)Here, Q: the flow rate, Sens: an evaluation constant, Vu: the upstream side voltage, Vd: the downstream side voltage, and (Vu−Vd)/(Vu+Vd): the sensor output.
To qualitatively describe Expression 1, (Vu−Vd) has a value that changes depending on the flow rate and temperature of the fluid flowing through the flow path, and (Vu+Vd) has a value that changes almost depending on the temperature, so that ((Vu−Vd)/(Vu+Vd)) is considered to change ideally depending on only the flow rate of the fluid.
Meanwhile, in practice, the effects of the type of measuring target fluid, an ambient temperature, the temperature of the fluid, and the like cause a zero point error or a span error in the flow rate calculated in accordance with Expression 1.
For example, even in a state where the fluid does not flow, when the ambient temperature changes, a zero point output of (Vu−Vd) changes, and the flow rate Q calculated in accordance with Expression 1 does not become zero.
For this reason, as disclosed in PTL 1 or the like, there is a flow meter in which a zero point correction function M including (Vu+Vd) as a temperature index is defined, and when fluid does not flow, a value of [((Vu−Vd)/(Vu+Vd))−M] is made zero independently of an ambient temperature to make a zero point correction.
However, the temperature index (Vu+Vd) used as a variable of the zero point correction function M has a linear characteristic only in some temperature band, and therefore even in the case of using the zero point correction method disclosed in PTL1, the zero point correction can be sufficiently made only in a range of 15° C. to 35° C., failing in the zero point correction in a wider temperature band such as a range of 15° C. to 60° C.
Also, the zero point correction function M is affected by a fluid type, and therefore different for each fluid type. Accordingly, it is necessary to preliminarily determine the zero point correction function M for each measuring target fluid type, taking time and effort very much for actual measurement. In other words, in the past, the relationship between the zero point correction function M and a fluid type has not been well known, and therefore an accurate correction has not been able to be made without doing work such as accurately determining the zero point correction function M for each type.
Such a problem may also occur when making a span correction of a flow rate calculated in accordance with Expression 1 or the like.